The present invention generally relates to liquid crystal display devices and more particularly to a high-contrast liquid crystal display device characterized by a fast response speed and a low electric power consumption.
FIG. 1 shows the construction of a conventional liquid crystal display device of the so-called TN-mode.
Referring to FIG. 1, the conventional liquid crystal display device includes a glass substrate 2a carrying thereon a number of active devices including pixel electrodes 6 and cooperating bus lines 5, wherein the glass substrate 2a faces a glass substrate 2b carrying thereon an opposing electrode 3, with a liquid crystal layer 1 interposed between the glass substrate 2a and the glass substrate 2b. It should be noted that the glass substrate 2a further carries a molecular alignment film 4 so as to cover the foregoing active devices, while the glass substrate 2b carries another molecular alignment film 5 so as to cover the opposing electrode 3.
In the conventional structure of FIG. 1, a liquid crystal called TN (twist-nematic) type is used commonly for the liquid crystal layer 1. In such a conventional, TN-mode liquid crystal display device using a TN-type liquid crystal, the liquid crystal molecules are aligned generally parallel to the plane of the substrates in the non-activated state thereof in which no drive voltage is applied to the liquid crystal layer. In the non-activated state, the liquid crystal molecules are further twisted between the substrate 2a and the substrate 2b with a twist angle of 90xc2x0. When a drive voltage is applied to the liquid crystal layer 1, on the other hand, the liquid crystal molecules are aligned generally perpendicular to the plane of the substrates 2a and 2b. 
Such a TN-mode liquid crystal display device is used commonly in various information processing apparatuses. Further, low-cost fabrication process of such a TN-mode liquid crystal display device is well established by now.
On the other hand, a TN-mode liquid crystal display device generally has a drawback in that the contrast ratio of represented images changes substantially depending on the viewing angle. While there are various attempts to improve the viewing angle characteristic of TN-mode liquid crystal display devices, it has been still difficult to realize a viewing characteristic comparable to that of a CRT display device.
On the other hand, there is another type of liquid crystal display device in which the liquid crystal molecules are aligned generally perpendicularly to the plane of the glass substrate. In such vertically aligned liquid crystal display devices, the liquid crystal molecules are aligned generally perpendicular to the plane of the glass substrates in the non-activated state.
FIGS. 2A and 2B show the construction of one type of such a vertically aligned liquid crystal display device.
Referring to FIG. 2A showing a pixel of such a vertically aligned liquid crystal display device in the non-activated state thereof, the liquid crystal display device includes a first glass substrate 10 carrying thereon a pair of electrodes 11a and 11b and a second glass substrate 12 facing the first glass substrate 10, and a liquid crystal layer 14 is sandwiched between the glass substrate 10 and the glass substrate 12. In the non-activated state of the liquid crystal display device, it should be noted that no drive voltage is applied across the electrodes 11a and 11b. 
The liquid crystal layer 14 includes liquid crystal molecules 16, wherein the liquid crystal molecules 16 are aligned generally perpendicularly to the plane of the substrate 10 in the non-activated state of the liquid crystal display device represented in FIG. 2A. It should be noted that the surface of the substrate 10 on which the electrodes 11a and 11b are provided is covered by a molecular alignment film not illustrated. Similarly, the surface of the substrate 12 facing the liquid crystal layer 14 is covered by a molecular alignment film not illustrated. Further, a pair of polarizers not illustrated are disposed at respective outer-sides of the glass substrate 10 and the glass substrate 12.
In the activated state represented in FIG. 2B in which a drive voltage is applied across the electrodes 11a and 11b, on the other hand, the liquid crystal molecules 16 are aligned in the direction of the electric filed inside the liquid crystal layer 14. Thereby, the pixel represented in FIG. 2B is divided into a first region at a first side of a line A-Axe2x80x2 and a second region at a second, opposite side of the line A-Axe2x80x2, wherein it can be seen that the liquid crystal molecules 16 are tilted in respective, mutually opposite directions in the first region and in the second region. As a result of such a subdivision of the pixel, the liquid crystal display device provides an excellent viewing angle characteristic.
On the other hand, the vertically aligned liquid crystal display device of FIG. 2 has a drawback in that it requires a drive voltage of at least 5 V. In order to reduce the power consumption of the liquid crystal display device, it is desired to reduce the drive voltage.
In a liquid crystal display device, the drive voltage is generally reduced by increasing the retardation value xcex94nxc2x7d, wherein xcex94n represents the birefringence and d represents the cell thickness. On the other hand, there has been little information about the optimum value for the birefringence xcex94n or for the cell thickness d in this type of the vertically aligned liquid crystal display devices.
Further, this type of vertically aligned liquid crystal display devices have conventionally suffered from the problem of poor response speed. This drawback becomes particularly conspicuous when performing a motion picture representation.
Accordingly, it is a general object of the present invention to provide a novel and useful liquid crystal display device wherein the foregoing problems are eliminated.
Another object of the present invention is to provide a liquid crystal display device, comprising:
a first substrate;
a second substrate facing said first substrate;
a liquid crystal layer interposed between said first and second substrates; and
a group of electrodes disposed on said first substrate so as to create an electric field in said liquid crystal layer generally parallel to said first substrate in an activated state in which a drive voltage is applied to said group of electrodes;
said liquid crystal molecules aligning generally perpendicularly to a plane of said first substrate in a non-activated state in which said drive voltage is not applied to said group of electrodes, said liquid crystal molecules aligning generally parallel to said plane of said first substrate in said activated state;
said liquid crystal molecules having a pre-tilt angle of less than 90xc2x0 in at least one of a part of said liquid crystal layer corresponding to a pixel and said electrode on said first substrate.
According to the present invention, the response speed of the liquid crystal display device is improved by locally setting the pre-tilt angle of the liquid crystal molecules to be less than 90xc2x0. Thereby, such pre-tilted liquid crystal molecules act as a nuclei when a drive electric field is applied to the liquid crystal layer, and the tilting of the liquid crystal molecules propagates rapidly throughout the liquid crystal layer, starting from such a site of the pre-tilted molecules. Associated with this, the drive voltage of the liquid crystal display device is reduced, and hence the electric power consumption.
Other objects and further features of the present invention will become apparent from the following detailed description when read in conjunction with the attached drawings.